Extrinsic Fabry-Perot interferometers are used to measure quantities such as strain, temperature, pressure, and displacement. Interferometers of this type are advantageous for use in embedded sensing applications for a number of reasons. For example, extrinsic Fabry-Perot interferometers offer the high sensitivity typical of an interferometer while at the same time overcoming many of the common drawbacks associated with interferometers. In particular, extrinsic Fabry-Perot interferometers are insensitive to polarization and have good thermal stability. The thermal drift of these interferometers is expected to be about 0.0002 fringes per 100 C.
One problem encountered when Fabry-Perot interferometers are employed as sensors relates to the known methods for analyzing the output data. A simple well-known output method is fringe counting, but this method does not provide absolute, unambiguous data. In particular, fringe-counting can only measure changes in sensor length and it cannot distinguish between tension and compression. Accordingly, while fringe counting may be adequate in a controlled laboratory environment, the technique is inadequate for more typical environments.
Therefore, given the problems associated with the known methods for analyzing data from a Fabry-Perot interferometer, there is a need for a technique to absolutely measure environmental parameters such as sensor strain.